It is well known that one of the dangers in the operation of automotive vehicles is the locking of the braked wheels so that effective braking of the vehicle cannot continue and a dangerous situation may arise because the locked wheel tends to skid on the road surface.
Put otherwise, for four-wheel passenger vehicles, tractors, aircraft and, indeed, any vehicular system in which a wheel engages the ground with rolling friction, effective braking and control of the vehicle is only possible as long as the wheel continues to roll along the ground or any interruption of the rolling during braking is at best momentary. Should the wheel lock, while the vehicle still has continuous momentum, uncontrolled sliding friction develops and skidding occurs.
The effect is most noticeable upon icy surfaces where locking of the wheel, while the vehicle has considerable momentum, represents practically complete loss of control since the sliding friction between the wheel and the ground surface is a minimum.
An experienced vehicle operator is able to avoid such situations or successfully extricate himself from such situations by avoiding overbraking entirely but, if such overbraking should occur, by briefly relieving the wheel-brake pressure and thus engaging in a stepped braking action. Since such operations require great sensitivity on the part of the vehicle operator and the instinct of the operator is not always sufficient, automatic systems have been developed to assist the operator in antiskid control of vehicle brakes. In these systems, the locking of the wheel is generally detected and the pressure in the wheel-brake cylinder may be relieved to undo the locking of the wheel.
These systems have not, however, been fully satisfactory, especially because they tend to generate a vibratory action which coincides with resonance of the vehicle and builds up undesirable vibrations in the vehicle.
In other cases, the braking force controller tends to react upon the braking lines and the wheel brake cylinder, creating considerable discomfort for the operator.